1. Field of Invention
The present invention belongs to the technological fields of a driving circuit including a data line driving circuit for driving an electro-optical device, such as a liquid-crystal apparatus of an active-matrix transistor driving method, such as a thin-film transistor (hereinafter referred to as a xe2x80x9cTFTxe2x80x9d where appropriate), and an electro-optical device of a type incorporating such a driving circuit. More particularly, the present invention belongs to the technological fields of a driving circuit for an electro-optical device, which adopts a driving method for driving plural data lines simultaneously in order to support high dot frequencies and color image signals, and an electro-optical device of a type incorporating such a driving circuit.
2. Description of Related Art
This type of driving circuit for an electro-optical device includes a data line driving circuit, a scanning line driving circuit, and a sampling circuit and the like, which are used to supply image signals and scanning signals at a predetermined timing to data lines and scanning lines wired in an image display area of an electro-optical device.
Such a driving circuit is constructed so that when a line sequential driving method is adopted, image signals which are supplied to one image signal line from an external source are sampled by plural sampling switches provided in such a manner as to correspond to each data line, respectively, in accordance with a sampling control signal which is supplied in sequence in such a manner as to correspond to each data line from the data line driving circuit, and are supplied to each data line based on the line sequence. Also, generally, the data line driving circuit includes a shift register circuit including plural arranged latch circuits which output a transfer signal in sequence according to a reference clock. Furthermore, the construction is formed in such a way that a buffer circuit is interposed between this latch circuit and the sampling circuit, and the waveform of the transfer signal is shaped to become the sampling control signal, and even if the driving performance of the latch circuit is not sufficient to drive the sampling switch, the load of the sampling switch can be sufficiently dealt with by the buffer circuit.
Here, in response to the demand for higher quality of display images in recent years, the dot frequency in an electro-optical device, such as a liquid-crystal device, is becoming increasingly higher, for example, as in an XGA method, an SXGA method, or an EWS method. When the dot frequency is increased in this manner, the sampling performance in the sampling switch becomes insufficient, and the delay time in each TFT, which is an element of the driving circuit, exerts an adverse influence upon the quality of the display image. For example, a problem arises in that an image signal for the previous data line is written into the next data line, causing ghost or crosstalk. However, if the performance of the sampling switch and each TFT is increased to deal with this problem, a substantial increase in cost will occur.
For this reason, recently, a technology described below has been developed. For example, an image signal is converted from serial into parallel form in advance so that the image signal is divided into plural parallel image signals, or the image signal is divided into parallel image signals for each color in the case of a color image signal, after which the image signals are supplied to plural image signal lines provided in an electro-optical device. In the sampling circuit, plural parallel image signals which are converted from serial into parallel form are sampled simultaneously, and are supplied to a plurality (for example, 6, 12, 24 lines, and the like) of data lines at the same time. According to this technology, since the time each sampling switch performs sampling can be increased about n times according to the number of data lines n which are driven simultaneously, the driving frequency in the driving circuit can be substantially decreased to about 1/n. That is, there is no need to improve the performance itself of the sampling switches and each TFT as described above, and it is possible to cope with a high dot frequency.
In a case in which plural data lines are driven simultaneously in this manner, since a sampling control signal is supplied simultaneously or the same sampling control signal is supplied to plural sampling switches, the data line driving circuit requires driving performance capable of withstanding a total of loads of the plural sampling switches. That is, the driving performance of the buffer circuit interposed between the latch circuit and the sampling switch must be increased according to the total of loads of the plural sampling switches. For this purpose, the size of the TFT which is an element of the inverter included in the buffer circuit need only be increased. However, if the size of the TFT is simply increased, there occurs the need to increase the driving performance in the latch circuit for driving this TFT by a transfer signal, causing the power consumption in the shift register circuit in which, in particular, the large amount of the power consumption is conventionally deemed to be problematical in the field of the relevant electro-optical device, to be increased even more. Accordingly, a construction is generally adopted in which the buffer circuit is formed of inverters of plural stages which are connected in series so that the driving performance in the buffer circuit is increased in a stepped manner for each inverter. That is, a construction is adopted in which the size of the TFT which is an element of an inverter of a stage on the side of the latch circuit of the buffer circuit is small and the size of the TFT which is an element of an inverter of a stage on the side of the sampling switch of the buffer circuit is large.
On the other hand, an electro-optical device of a driving circuit built-in type has been developed in which a driving circuit such as that described above is provided on a substrate which is an element of the main unit of an electro-optical device, such as a liquid-crystal device. This electro-optical device of a driving circuit built-in type is advantageous in achieving an overall reduction in size of the device and a decrease in cost in comparison with an electro-optical device of a type in which a driving circuit is formed on a separate substrate and is provided externally.
However, if the above-mentioned buffer circuit formed of plural stages is provided in the above-mentioned liquid-crystal device of a driving circuit built-in type, an increase in the occupied area by the buffer circuit having a larger size on the substrate of a liquid-crystal device, and the like, becomes a problem. In particular, as in the above-mentioned conventional liquid-crystal apparatus of a line sequential driving method, if each inverter is formed of TFTs extending in a longitudinal direction along the data lines and this is connected in series in a longitudinal direction at plural stages along the data lines, conventionally, there is the problem in that the ratio of the ineffective use area by the buffer circuit, which occupies an area on a horizontally elongated substrate along the scanning lines present between the image signal lines and the shift register circuit, is increased. Ultimately, a non-image display area for forming a data line driving circuit in the upper or lower portion of the image display area is extended, resulting in a problem in that a situation is brought about which is contrary to a general demand for a smaller size and a lighter weight of the overall device and a larger area of the image display area of the same device size in the technological field of the relevant electro-optical device.
The present invention has been achieved in view of the above-described problems. A driving circuit for an electro-optical device is provided, which is capable of achieving a smaller size of the device or a larger size of the image display area of the same device size by efficiently using an area on a substrate in an electro-optical device such as a liquid-crystal device, which is a driving circuit built-in type and which adopts a driving method for driving plural data lines simultaneously, and to provide an electro-optical device incorporating the driving circuit.
To solve the above-mentioned problems, the driving circuit for an electro-optical device in accordance with the present invention is a driving circuit for an electro-optic device including an electro-optical material sandwiched between a pair of substrates, and plural data lines and plural scanning lines which intersect each other on one substrate of the pair of substrates, the driving circuit comprising: plural sampling switches provided on one of the substrates, for sampling image signals in accordance with a sampling control signal and for supplying the image signals to the plural data lines, respectively, and a data line driving circuit that supplies the sampling control signal simultaneously to each group of sampling switches connected to n (n is an integer of 2 or more) data lines adjacent to the plural sampling switches, the data line driving circuit comprising a shift register circuit that sequentially outputs a transfer signal from each of a plurality of latch circuits, and a buffer circuit that outputs the transfer signal as the sampling control signal, and at least one transistor of the buffer circuit extends in a same direction as a direction in which a width of the channel thereof intersects the data lines on one of the substrates.
According to the driving circuit for an electro-optical device in accordance with the present invention, a sampling control signal is supplied by the data line driving circuit to n sampling switches simultaneously to each group of sampling switches connected to n adjacent data lines. At this time, in the data line driving circuit, a transfer signal is output in sequence by a shift register circuit, and this transfer signal is output as the above-mentioned sampling control signal via a buffer circuit. Then, an image signal is sampled by each sampling switch in accordance with the sampling control signal and is supplied to the plural data lines, respectively. In this manner, by driving the plural sampling switches simultaneously, it is possible to drive the data lines in such a manner as to correspond to an image signal having a high dot frequency as in, for example, XGA, SXGA, and EWS.
Here, in particular, in at least one of the transistors included in the buffer circuit, the direction of the channel width is in a direction (for example, in a direction parallel or nearly parallel to the scanning lines) intersecting the data lines on one of the substrates. Therefore, in the present invention, it is possible to provide a transistor having a wide channel width (that is, of a large size having a high driving performance capable of driving a sampling circuit having a larger load) in comparison with a case in which a transistor which is an element of the inverter is disposed so that its channel width is within the width (that is, the pitch of the data lines) of one data line as in a buffer circuit including an inverter in such a manner as to correspond to each latch circuit, in a conventional line sequential driving method.
Alternatively, it is possible to provide a TFT having a large channel width and having a large size which may be used for an inverter within a longitudinal region parallel to the data lines on the substrate in comparison with a case in which a TFT which is an element of the inverter is disposed so that the direction of its channel width coincides with the longitudinal direction parallel to the data lines and is within the pitch of the data lines as in a buffer circuit including an inverter, to correspond to the output of a shift register in the conventional line sequential driving method.
In one embodiment of the present invention, the channel of the transistor has a width within the pitch of the adjacent 2 to n data lines.
According to this embodiment, in the conventional line sequential driving method, a vertically elongated transistor corresponding to the pitch of the data lines is laid out on a substrate. However, in the present invention, by setting the direction of the channel width in a direction intersecting the data line while the channel width is within the total width of n data lines which are driven simultaneously and by effectively using the area on the substrate extending along its length along the scanning lines between the shift register circuit and the sampling circuit, it is possible to lay out a horizontally elongated transistor of a large size corresponding to the total width of the plural data lines on a substrate.
As a result of the above, according to the present invention, while effectively using the area on the substrate, it is possible to provide a buffer circuit including an inverter formed of a large transistor capable of driving a load even if the load in the sampling circuit is increased with an increase in the number of data lines which are driven simultaneously, and it is possible for the relevant driving circuit having saved space to perform a satisfactory driving operation even in the case of a high dot frequency.
In one embodiment of the driving circuit for an electro-optical device according to the present invention, the buffer circuit includes inverters of m (m is an integer of 2 or more) stages which are connected in series in such a manner as to correspond to each of the latch circuits.
According to this embodiment, by increasing the size of the transistor which is an element of an inverter of each stage in a stepped manner of the inverters which are of m stages, it is possible to increase a load in the sampling circuit, which can be driven by all the inverters. That is, it is possible to increase the number of sampling switches which can be driven simultaneously.
Therefore, since a relatively small transistor, which is an element of the inverter of the first stage when viewed from the side of the latch circuit, is required, the size of the transistor which is an element of the latch circuit which inputs a transfer signal to this transistor can also be required to be small. For this reason, a lower power consumption in the shift register circuit comprising plural latch circuits can be achieved.
However, if the number of stages (m) of the inverters is increased, the total of the delay time by the transistor which is an element of these inverters is also increased. Therefore, in practice, this number of stages (m) of the inverters is determined by considering the dot frequency, required specifications, image quality, and the like, so that the total of this delay time ultimately does not exert an adverse influence upon the display image.
In this embodiment, the channel width of the transistor possessed by the (i+1)-th stage counting from the side of each of the latch circuits may be set larger than the channel width of the transistor possessed by the inverter of the i-th stage.
With such a construction, since the size of the transistor which is an element of an inverter of each stage is increased in a stepped manner, it is possible to increase the load in the sampling circuit which can be driven by all the inverters, making it possible to increase the number of sampling switches which can be driven simultaneously.
In an embodiment in which this buffer circuit includes inverters of m stages, the inverters of m stages are provided in a meandering shape, with a first portion extending in a first direction intersecting the data lines from a side near the shift register circuit and a second portion extending in a direction opposite to the first direction from the first portion and may be arranged in sequence in a direction intersecting the scanning lines.
With such a construction, it is possible to take a wider channel width of the transistor, which is an element of the inverter, by an amount corresponding to the meandering. For example, if the inverters are provided in a meandering shape of a letter S, a channel width can be secured which is approximately three times wider than that in a case in which a channel width is simply taken straight in a first direction, thereby making it possible to increase the driving performance of the transistor according to an increase in the channel width.
In this case, furthermore, a power wiring extending in the first direction may be shared between the first and second portions.
With such a construction, since the power wiring extending in the first direction is shared between the first and second portions, it is possible to shorten the length in a direction (for example, in a longitudinal direction along the data lines) at right angles to the first direction in the entire buffer circuit by an amount corresponding to the width of the power wiring to be shared in comparison with a case in which the power wiring is not shared.
In another embodiment of a driving circuit for an electro-optical device in accordance with the present invention, the buffer circuit includes an inverter of one stage in such a manner as to correspond to each latch circuit, respectively.
According to this embodiment, since the inverter which is an element of the buffer circuit is of one stage, the delay time of the entire buffer circuit is completely or nearly equal to the delay time in the transistor which is an element of the relevant inverter of one stage. For this reason, a shorter delay time results in comparison with a case in which plural inverters are provided and the delay time is added in series.
In this embodiment, the inverter of one stage may comprise plural inverters which extend in directions intersecting the data lines, respectively, and which are connected in parallel in such a manner as to be arranged in sequence in directions intersecting the scanning lines.
With such a construction, since the inverter of one stage comprises plural inverters which are connected in parallel and which are arranged in sequence in directions (for example, in directions parallel to or nearly parallel to the data lines) intersecting the scanning lines, it is possible to effectively use the area on the substrate having an area corresponding to the total width of the data lines which are driven simultaneously and to lay out the relevant inverter.
In this case, furthermore, a power wiring extending in a direction intersecting the data lines may be shared between the plural inverters which are connected in parallel.
With such a construction, since a power wiring extending in a direction intersecting the data lines is shared between the plural inverters which are connected in parallel, it is possible to shorten the length in a direction (for example, in a direction parallel to or nearly parallel to the data lines) intersecting this direction in the entire buffer circuit by an amount corresponding to the width of the power wiring to be shared in comparison with a case in which the power wiring is not shared.
In yet another embodiment of a driving circuit for an electro-optical device in accordance with the present invention, the transistor comprises a complementary transistor.
According to this embodiment, the complementary transistor makes it possible to increase the input impedance of each inverter, making it possible to drive a sampling switch having a large load via the relevant complementary transistor in accordance with a transfer signal from a latch circuit having a small driving performance.
In still another embodiment of a driving circuit for an electro-optical device in accordance with the present invention, the data line driving circuit further comprises a phase adjustment circuit for limiting a signal width of the transfer signal to a predetermined value in each section between the latch circuit and the buffer circuit.
According to this embodiment, since the signal width (the time in which the signal is assumed to be at a high level) of the transfer signal is limited to a predetermined value (predetermined time width) by the phase adjustment circuit present between the latch circuit and the buffer circuit, the overlap between the transfer signals which are output almost simultaneously from the latch circuit is reduced. Consequently, crosstalk and ghost, which occur due to such overlapping, between the data lines (that is, every n data lines) which are driven almost simultaneously, can be prevented.
In still another embodiment of a driving circuit for an electro-optical device in accordance with the present invention, plural image signal lines are arranged along the scanning lines on one of the substrates, and the buffer circuit is formed in an area on the substrate between the plural image signal lines and the shift register circuit.
According to this embodiment, the sampling circuit samples an image signal supplied to the plural image signal lines in accordance with a sampling control signal. Here, since the buffer circuit is formed in an area on the substrate between the plural image signal lines and the shift register circuit, effective use of the area on the substrate can be achieved by disposing a horizontally elongated inverter in a horizontal rectangular area along the image signal lines and the scanning lines.
In still another embodiment of a driving circuit for an electro-optical device in accordance with the present invention, the image signal is subjected to n serial-to-parallel conversions, and is supplied to the sampling circuit via n image signal lines.
According to this embodiment, the image signal is subjected to n serial-to-parallel conversions, and is supplied to the sampling circuit via the n image signal lines. Therefore, even when the dot frequency is high as in, for example, XGA, SXGA, or EWS, high-quality image display is made possible by serial-to-parallel conversion even by using a sampling circuit having a relatively low sampling performance or having a relatively low performance in delay time, and the like.
An electro-optical device in accordance with the present invention comprises the above-described driving circuit for an electro-optical device of the present invention.
According to the electro-optical device in accordance with the present invention, since the electro-optical device comprises the above-described driving circuit of the present invention, it is possible to miniaturize the entire device and to increase the size of the image display area in a device of the same size, and at the same time, an electro-optical device, such as a liquid-crystal device, capable of displaying a high-quality image, can be realized.
In one embodiment of an electro-optical device in accordance with the present invention, on one of the substrates, plural pixel electrodes disposed in a matrix, and plural transistors for driving the plural pixel electrodes, respectively, are further provided, and the plural data lines and the plural scanning lines are connected to the plural transistors, respectively.
According to this embodiment, an electro-optical device, such as a liquid-crystal device, can be realized using the commonly-termed xe2x80x9cTFT active-matrix driving methodxe2x80x9d, which is capable of displaying a high-quality image.
In order to solve the above-described problems, an electronic apparatus of the present invention comprises the above-described electro-optical device of the present invention.
According to this embodiment, it is possible to provide an electronic apparatus comprising an electro-optical device capable of displaying a high-quality image.
Such an operation and the other advantages of the present invention will become apparent from the embodiments described below.